The small mode areas for light propagating through fibers lead to high optical intensities even for moderate power levels. It is therefore no surprise that particularly a fiber input end, into which a laser beam is launched, can easily be destroyed, particularly when the fiber end is not very clean. The more surprising effect, however, is that a fiber can also burn down starting from the output end. Here, a bright white spot can be seen, which results from a hot plasma forming at the fiber end. As this plasma fuses the fiber, it propagates back towards the input end with a velocity which can be several meters per second. In standard single-mode fibers, this phenomenon can occur already for powers below 1 W if something (e.g. a burning dust particle) triggers the plasma at the output end. An important ingredient for the underlying mechanism is that the absorption losses of silica sharply rise at temperatures above 1000 °C.
The effect of fiber fuse can be disastrous in a telecom system where kilometers of fiber may be destroyed as a result of a failure at only one point. This is a concern for fiber-optic links with extremely high transmission capacities. A partial remedy may be to reduce the cladding diameter at certain points so that the fiber fuse is stopped at such locations .
Note that high-power fiber amplifiers are at particularly high risk of suffering fiber fuse; appropriate measures have to be taken to ensure device reliability.
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